106
VEGETABLE CELLS.
mucilage exhibits an equal density up to the border of thedrop of water, so that where the water commences themucilage appears as if cut off from it. Sometimes it isdenser at the borders of the drop of water, and seems toform a proper membrane around it. Probably this is theconsequence of a lengthened action of the water upon themucilage. In any case the watery vesicles go no further;certainly undergo no organic changes.
All perfect nuclei lying in the fluid must also be ex-cluded in the consideration of cell-formation. They arewholly absent at first, and multiply in proportion to thenumber of fully-developed endosperm-cells contained inthe embryo-sac. They are the nuclei of these cells, andbecome free, with the granular contents, through theirdestruction. These nuclei are very apt to lead the ob-server astray, since they are not capable of the subsequentstages of development.
Small, globular drops of perfectly homogeneous mucilage,with defined outline, are absolutely constant phenomena inthe process of cell-formation in the embryo-sac. Theyvary from •001--004 of a line, and are distinguished fromthe mucilage-granules, even in the earliest condition,by a perfectly globular form and smooth surface. Themucilage-globules are never absent; they always con-stitute the first stage of cell-formation (pi. II, fig. 7, a,b, c ; 8, a). They exactly resemble the mucilage-globuleswhich represent the nascent germ-cells in Valonia andother Algae , in Lichens and Fungi.
A more advanced stage is represented by the largermucilage-globules, in which a smallish ring is inclosed,while the whole globule possesses an uniform consistence(fig. 7, d, e, /). The inclosed ring may appear clearer(fig. 9, a) or denser (fig. 9, b) than the outer part of theglobule. There can be no doubt, and the further deve-lopment also confirms it, that the mucilage-globule is acell-nucleus, the inclosed ring a nucleolus. How havethe nucleus and nucleolus originated ? This is a questionwhich I cannot answer from direct observation on cell-